/* reverse filename -- reverses a file in place */

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>

/* Print error message and die */
void error(char* msg) {
  perror(msg);
  exit(1);
}

/* File name in argv[1] */
int main(int argc, char** argv) {
  char *buf;  /* Array holding file content */
  int size, fd;  /* size of buf, file descriptor */
  char *p1, *p2;

  /* Read file into buf */
  if (argc<2) error("no filename");
  fd=open(argv[1],O_RDWR);
  if (fd==-1) error(argv[1]);
  size=lseek(fd,0,SEEK_END);  /* get size */
  if (size==-1) error("lseek");
  buf=(char*)malloc(size);
  if (buf==0) error("out of memory");
  if (lseek(fd,0,SEEK_SET)) error("lseek");
  if (read(fd,buf,size)!=size) error("read");

  /* Reverse buf */
  for (p1=buf,p2=buf+size-1; p1<p2; ++p1,--p2) {
    *p1^=*p2;
    *p2^=*p1;
    *p1^=*p2;
  }

  /* Write buf back to file */
  if (lseek(fd,0,SEEK_SET)) error("read");
  if (write(fd,buf,size)!=size) error("write");
  if (close(fd)==-1) error("close");
  return 0;
}